This invention relates to a head wire constituting an impact dot matrix print head and a process for its production.
An impact dot matrix printer head is used to print characters by making head wires project using the attraction generated by an electromagnet (the clapper type) or the force-releasing elastic energy stored in a spring (the spring charge type), and by conveying the force to the paper through an ink ribbon to transfer the ink onto the paper.
As FIGS. 1 and 2 show, the print head 1 of the spring charge type is manufactured by brazing a specified number of head wires 2 onto armatures 4 through bars 3, and by welding these armatures 4 to springs 5. Furthermore, the springs 5 are built into the magnetic circuit together with yokes 7 with a predetermined offset to core 6.
The armature 4 is always attracted toward the core 6 by the permanent magnet 8, and accumulates elastic energy. By exciting the coil 9 in synchronization with the printing timing, the magnetic field of the permanent magnet 8 is canceled, and the resilience energy stored in the spring 5 is converted into kinetic energy. Due to this, the head wire 2 is projected through the wire guide 10a at the end of the head, and transfers ink onto the paper 12 on the plates 16 through the ink ribbon 11. Here, the distance of the movement of the head wire 2 is about 0.25 to 0.40 mm.
In such a print head 1, as FIG. 3 shows, the end of the head wire 2 is caved into the ink ribbon 11 and the edge is worn out by abrasion after repeated printings. As wear advances further, the distribution of contact pressure with the ink ribbon 11 increases in the central part, and finally, the central part of the head wire 2 becomes worn out, and the length of the head wire 2 is decreased by the amount of wear w.
Thus, the shortened head wire 2 increases the gap with the paper 12, lowering the printing pressure. Since the end of the head wire 2 is rounded, the printing dot diameter decreases, and the printing becomes less dense and more unclear.
Furthermore, a head wire 2 with a rounded end may pierce the fabric of the ink ribbon 11, breaking the ink ribbon 11 easily, and the end of the head wire 2 may break if the head wire 2 is caught by the ink ribbon 11.
To decrease the wear of the end of the head wire, harder materials have been used for the head wire, or "dye ink" has been used as the ink impregnated in the ink ribbon.
When hard tungsten carbide (cemented carbide) is used as the material for the head wire, although a decrease in wear can be realized, the mass of the head wire increases and cannot keep up with increased printing speeds. In spite of its hardness, it has low tenacity, resulting in easy breakdown of the head wire.
Since the main cause of the wear of the end of the head wire is abrasive wear due to the carbon black pigment contained in the ink, the pigment content has been minimized while the content of the dyes has been increased, preventing the wear of the end of the head wire. This is called "dye ink", while conventional ink is called "pigment ink" to discriminate between them.
Although the use of this "dye ink" decreases the wear of the head wire, printing is less dense and more unclear than printing with "pigment ink", which excels in clearness, longevity and quick drying properties, and the like of the ink ribbon is shortened.
The differences in head wire wear between the head wire materials and between inks impregnated in the ink ribbon are shown in FIG. 4. This graph shows the results of tests using an IBM 5577 printer.
By the above mentioned reasons, in the print head for high speed printing a high speed steel with quick response and high tenacity is used as the head wire, and in the ink ribbon a dye ink is used as the ink impregnated.
However, even if dye ink is used, the wear of the end of as head wire made of high speed steel is still large, and the head gap must be readjusted periodically. For applications requiring a high volume of printing, the print head itself must be replaced.
Furthermore, as a result of using dye ink, the printing is thin and unclear, and the life of the ink ribbon is short.
To decrease the wear of the end of the head wire and to solve the above problems, methods have been proposed in which a cemented carbide tip is fixed on the end of a head wire made of high speed steel, or in which a head wire made of high speed steel is covered with a cemented carbide ring; however, all of these methods have problems from the view point of reliability, processability and production costs, and have been used practically.